Nowadays, household refrigerators and similar appliances generally adopt a vapor compression refrigerating cycle in which a CFC (chlorofluorocarbon)-based refrigerant is circulated. However, CFC-based refrigerants, when released into the atmosphere, reach the stratosphere without being decomposed, and destroy the ozone layer, constituting a much talked-about environmental problem. For this reason, production and use of conventionally widely used CFC-based refrigerants and alternatives thereto such as R-134a are becoming increasingly restricted worldwide.
Against this background, a reassessment has been made of the advantages of the thermodynamic cycle that has long been known as the reversed Stirling cycle, and in recent years much attention has been paid to Stirling refrigerating systems that exploit it. However, most Stirling refrigerating devices that have been put into practical use thus far are for use in comparatively small Stirling refrigerators with refrigeration performance of a few tens of watts or lower; that is, no Stirling refrigerating devices have ever been put into practical use that have refrigeration performance of the order of a few hundred watts, for which the highest demands are expected as models for household and commercial use.
Now, the conventional Stirling refrigerator disclosed in Japanese Patent Registered as No. 2714155 will be described. The refrigerator is provided with a Stirling refrigerating device, with a cold-side heat exchanger placed in a cold air passage formed in a rear portion of a refrigerating chamber of the refrigerator so as to allow circulation of cold air, and with a hot-side heat exchanger connected to a metal superficial portion of a body of the refrigerator.
When the Stirling refrigerating device is operated, cold is generated by the cold-side heat exchanger, and the cold air circulated through the cold passage cools the interior of the refrigerator. Moreover, an air-cooling fan is provided in a V-shaped heat rejection passage formed at the back of the cold air passage so that the heat accumulated in the hot-side heat exchanger is actively rejected out of the refrigerator. Furthermore, part of the heat rejected from the hot-side heat exchanger is rejected out of the refrigerator also by way of the metal superficial portion of the refrigerator body. This alleviates the load on the hot-side heat exchanger and thereby enhances heat rejection efficiency.
This conventional refrigerator is built as a system in which cooling is achieved by discharging cold as sensible heat directly into the refrigerator by using air as a medium. Accordingly, to obtain refrigeration performance of the same order as achieved with this conventional vapor compression refrigerating cycle exploiting sensible heat, it is necessary to use large heat exchangers, making the system bulky. Thus, with this conventional construction, it is difficult to achieve satisfactory miniaturization and cost reduction required in systems for household use.
The biggest stumbling block is miniaturization of the refrigerating system. In particular, to secure storage space comparable with that permitted by the conventional vapor compression refrigerating cycle, it is essential to miniaturize the Stirling refrigerating device itself In recent years, miniaturization of Stirling refrigerating devices has been studied eagerly. As Stirling refrigerating devices are made increasingly compact, their heat rejecters and heat absorbers are made increasingly small, and the spaces inside their cylinders, filled with a working medium such as helium, are made increasingly small.
Accordingly, to obtain a large quantity of cold efficiently from a miniaturized Stirling refrigerating device, it is necessary to increase the heat exchange efficiency of the heat exchangers attached to its heat rejecter and heat absorber. As a result, despite the miniaturization of the Stirling refrigerating device itself, the heat exchangers attached thereto are now larger. This hinders satisfactory miniaturization of the refrigerating system as a whole.
Therefore, to realize a space-saving refrigerating system with desired refrigeration performance, it is very important, in addition to miniaturizing the Stirling refrigerating device itself, to miniaturize the heat exchangers while maintaining their heat exchange efficiency.
Incidentally, in a conventional medium-size household refrigerator exploiting the conventional vapor compression refrigerating cycle, piping for heat rejection, as long as about 20 m including a condenser, is laid in a serpentine layout. Here, heat exchange is achieved by exploiting both the sensible heat of the refrigerant circulated through that piping and the latent heat resulting from condensation.
By contrast, in the conventional Stirling refrigerator disclosed in the Japanese Patent mentioned above, forced air-cooling is proposed, which is considered to require an extremely large heat exchanger for heat rejection. Thus, to achieve satisfactory heat rejection from the heat-rejecting heat exchanger, it is necessary to increase the amount of cooling air blown onto the heat-rejecting heat exchanger. Increasing the amount of cooling air, however, results in increasing the electric power consumption of the blower fan. This means that extra electric power consumption degrades the efficiency of the system as a whole.
Moreover, in this conventional Stirling refrigerator, the heat-rejecting head of the Stirling refrigerating device is connected to the metal superficial portion of the refrigerator body so that this portion plays a part in heat rejection and thereby alleviates the load of heat exchange on the heat-rejecting heat exchanger. However, considering the properties of the metal material used in the superficial portion of the body and the ambient conditions under which heat is rejected, the metal superficial portion exhibits high thermal resistance in the directions in which heat diffuses. Thus, the metal superficial portion contributes to effective heat rejection only in a portion thereof near the source of heat, i.e., near the heat-rejecting head. That is, the metal superficial portion has very little heat exchange effect, and thus contributes very little to the alleviation of the load on the heat-rejecting heat exchanger. This hinders satisfactory miniaturization of the heat-rejecting heat exchanger.
Moreover, to secure air-tightness inside the refrigerator, a door gasket made of flexible rubber or the like is laid along the edges of that side of the door which faces the interior of the refrigerator. As the door is opened or closed or in other situations, the cold air inside the refrigerator makes direct contact with the door gasket or the outer plate. This portion of the door thus tends to become especially cold as compared with the other portion thereof, and is therefore prone to collect dew resulting from condensation of moisture contained in outside air. When dew collects, it may drip to make the floor wet, or may produce rust in metal components.
To prevent this, it is common to embed a heater in the portion that is prone to collect dew so that the heater heats the portion to prevent dew from collecting. However, using a dew prevention heater results in extra electric power consumption unrelated to the operation of the refrigerating system, and is therefore disadvantageous in household refrigerators, which are expected to be low-priced and energy-saving.
On the other hand, drain water resulting from defrosting or the like of the interior of the refrigerator is collected in a drain pan. Since it is troublesome to periodically take out the drain pan to dispose of the collected water, it is customary to forcibly evaporate drain water by exploiting the heat generated by the condenser. This contributes to maintenance-free operation.
However, in a refrigerator exploiting the reversed Stirling cycle, there is provided no component that corresponds to a condenser, an integral part of the vapor compression refrigerating cycle. Thus, here, it is common to dispose of drain water by heating it with a heater. However, using a heater results in extra electric power consumption, inviting extra electricity charges, and is therefore uneconomical